1. Field of the Invention
The present invention relates to a light source device in which a laser diode is employed as a light source and used for a wireless optical communication.
2. Description of the Related Art
In recent years, there has been growing interest in wireless optical communications. There are several reasons for this as follows: the electromagnetic spectrum they use is not covered by current legislation, they are reliable systems, and they are not expensive. Moreover the possibility of an information leakage to the external due to rectilinear propagation property of the light is low and secrecy is high. The most widespread example of this type of communications can be found in the great majority of remote controls for electronic consumer goods. In this case the communication is usually unidirectional and very low-speed. Among the disadvantages of this type of device, the most important are the need for a visual link without obstacles and the limitation of the level of exposure to which the human eye can be subjected, which restricts the power and collimation characteristics of the bundle. This limitation has obliged the majority of these devices to use as radiation source an LED (Light Emitting Diode) or IRED (Infra Red Light Emitting Diode) instead of an LD (laser diode). The use of LEDs or IREDs has restricted in practice not only the irradiance (power per unit of area) of the bundle transmitted, but also the transmission speed, since the switching time of these devices (in the order of a few nanoseconds) is comparatively long if they generate radiant power of around tens of milliwatts or more, which is generally necessary for wireless optical communications. To these LEDs and IREDs diode, a laser diode is obviously advantageous with respect to switching time and generating radiant power, and is also advantageous in cost because it is popular as a device for optical discs such as MD, CD. Its only disadvantage is that it is necessary to modify the characteristics of the bundle (for example, in general, it is necessary to reduce the irradiance of the bundle) if there exists the possibility of its reaching the eyes, which is the case in most applications of wireless communications and illumination. The maximum permissible exposure level where this possibility exists is laid down in the European CENELEC EN 60825-1 (CEI 825-1:1993), CENELEC EN 60825-2 (CEI 825-2:1993), JISC6802:1997 and IEC60825-2:1993 standards relating to “Safety of laser products”. Maximum permissible exposure levels depend on several factors, important among which are radiation wavelength, duration and frequency of light pulses and type of source, extended or collimated.
In order to achieve that the irradiance is within the limits, it is always possible to use the solution of attenuating the bundle by means of an appropriate filter. However, this produces very high optical losses. There is also the possibility of reducing the irradiance and, moreover, taking advantage of this reduction to increase the cross-sectional size of the bundle or expand its angular divergence, or both, without causing high optical losses. When the bundle is expanded, its irradiance necessarily decreases, thus achieving the objective; furthermore, the power transmitted in the far field axial direction of the bundle can be increased.
Nevertheless, if this expansion is carried out by means of a conventional optical system, that is, through a combination of lenses and mirrors, the laser system is not eye-safe. This is due to the fact that viewing the bundle through optical instruments such as binoculars or telescopes is unsafe, since such instruments accomplish the inverse process to that of the expansion of the bundle, that is, they concentrate the laser bundle on the eyes, so that permitted exposure levels may be exceeded. Furthermore, this procedure for expanding the bundle reduces angular divergence, which is a disadvantage in those applications in which a wide pointing error tolerance is required.
Irradiance can also be reduced by means of diffusers. Diffusers achieve an increase in the angular divergence of the bundle without modifying the cross-section the bundle has when it reaches the diffuser. Obviously, mean radiance (power per unit of surface and unit of solid angle) of the bundle decreases, since the conservation of energy must be fulfilled. Where there is diffusion, the mean irradiance of the bundle decreases more rapidly with distance from the point of diffusion, since angular divergence is bigger than the one of the non-diffusion case. In this way it is possible to fulfil the requirements of CENELEC EN 60825-1(CEI825-1:1993), JISC6802:1997 and IEC60825-1:1993 with effect from a short distance away from the point of emission. Moreover, this solution reduces the problem of the possible viewing of the bundle with binoculars or telescopes, since, on reducing the mean radiance of the bundle, the maximum irradiance that can be achieved with such optical instruments is limited. This fact is related to loss of (spatial) coherence of the laser bundle after diffusion. These diffusers can be made in various ways. For example, a transmissive diffuser can be made with sheets of transparent material, one of whose surfaces is matt, that is, one of whose surfaces is such that the direction of the normal to the surface on which the refraction occurs can be considered as a random variable. From the probability distribution function of this normal and the distribution of radiant intensity of the laser bundle, it is possible to calculate the radiation intensity distribution after the diffusion. A more precise calculation would require the consideration of Fresnel reflections, which contribute to greater diffusion. If the irregularities are of the order of the wavelength, than it is necessary to use the wave optics theory to obtain the intensity distribution on the diffuser exit. This is the case of another type of diffuser that is made using holograms.
Reflection diffusers constitute a second type. A reflection diffuser can be achieved simply by using matte paint. The laser radiation impinges on the reflector with a small angular divergence and is reflected with a large angular divergence. A good diffusive reflector, such as those used as the inner coating of integrating spheres, produces a reflected intensity with a Lambertian pattern, that is, the reflector emits isotropically in the hemisphere it is facing.
The main problem of diffusers is that the only way to reduce irradiance is through expansion of the angular divergence, and this means that the divergence of the bundle is determined by the safety criteria. These are such that the irradiance achieved at a distance of 10 cm from the point of diffusion must be below the maximum permissible (10 cm is the minimum distance established by the regulations CENELEC EN 60825-1 (CEI825-1:1993), JISC6802:1997 and IEC60825-1:1993). If, in order to achieve this objective, it has been necessary to reduce the irradiance by a factor 1/C (C>1), then, and given that angular divergence does not vary, the reduction of irradiance at a distance D will be C=D2/(10 cm)2. This result is approximately correct when the divergence of the bundle is much greater than the solid angle subtended by the illuminated area of the diffuser at a distance of 10 cm, which is the case if the irradiance of the laser bundle has to be reduced. In this way, the transportation of light through the air involves high propagation losses, and this solution becomes useful only for small distances.
Diffusers have a second problem: Given the random nature of the principle of diffusion, intensity distributions, as a function of direction of emission, present gentle variations, and their form is not totally controllable (in the case of diffusers based on rough surfaces, these distributions tend to be Gaussian). In general, the desired type of intensity distribution is one that fulfils the eye-safety criteria within a given angular field of emission, and that emits nothing outside of it (so as not to lose laser power). This type of sharp distribution cannot be achieved with diffusers, which necessarily emit some power outside of the design angular field and, moreover, cannot maintain a constant intensity within that field. The overall result is a loss of transmission efficiency of the laser radiation.